Designers of automotive vehicle bodies seek structures that will enable the body to absorb low speed impacts with low repair costs while still absorbing much energy from higher speed impacts. For some vehicle applications it has been suggested to use a “crash box.” Crash box is the name applied to dedicated energy management units that are mounted between, and interconnect, the bumpers and longitudinal rails of a vehicle. Conceptually and theoretically, an ideal crash box is an inexpensive, low mass, easily replaceable unit that isolates and protects the rest of the vehicle structure from damage in crashes at velocities up to, e.g., 15 km/h. by deforming elastically (reversibly) at impact speeds below 8 km/h, and by deforming irreversibly (crushing) at force levels that are sufficiently high to dissipate the total impact energy at impact speeds between 8 and 15 km/h. Thus, crash boxes are intended to have minimal repair costs in crashes below 8 km/h, and, because they are easily replaceable “sacrificial” elements, to reduce vehicle repair costs in crashes between 8 and 15 km/h.
Since impact damage is to be confined to the crash boxes in impacts between 8 and 15 km/h, they are designed to crush at a lower force level than the rest of the vehicle body structure. However, this lower crush force level means that they are less efficient as energy absorbers in crashes above 15 km/h, i.e. that they dissipate less energy per unit length crushed than the body rails to which they are attached. For a vehicle of fixed length, from this standpoint of crushing at a lower force level and dissipating less energy per unit length than the rails to which they are attached, crash boxes are a less than optimum use of crush space. Thus, in addition to having a crash box that is easily repaired following a low speed impact, it would be desirable to have such a box that could be adjusted (tuned) to absorb a higher level of energy in a higher speed impact if, for example, a crash warning system on the vehicle sensed such an impending impact.
It can be visualized that repairable and tunable crash boxes could be based on either hydraulic or magnetorheological fluids (MRF). In such a unit, axial loading in a crash would cause a piston to stroke in a cylinder and dissipate energy either by forcing a viscous fluid through an orifice or shearing a fluid in the gap between piston and cylinder wall. A perceived advantage of such units is that they could, at least theoretically, have their crush forces adjusted/optimized to match the severity of each specific crash event based on sensor input as to the severity of the crash event. The adjustment could be through a rapid change of orifice size, or through a rapid change in the strength of the applied magnetic field in the case of an MRF. Another positive feature of such units is that the piston could be returned to its impact receiving position by a return force or mechanism such as a spring, so that the units would be fully restorable after a low speed crash. Thus such units could be both tunable and healable, at least for low speed crashes. However, such hydraulic and MRF units are quite heavy in practice and after having bottomed out in absorbing energy during an impact they are basically rigid units that resist any further crushing and in this way reduce the crush efficiency of the vehicle front end.
It is an object of this invention to provide a relatively simple and very adaptable design for tunable and repairable energy absorbing devices for an automotive vehicle based on the use of active materials as the energy absorbing element.